The leading cause of lower back pain arises from rupture or degeneration of lumbar intervertebral discs. Pain in the lower extremities is caused by the compression of spinal nerve roots by a bulging disc, while lower back pain is caused by collapse of the disc and by the adverse effects of articulation weight through a damaged, unstable vertebral joint. One proposed method of managing these problems is to remove the problematic disc and replace it with a porous device that restores disc height and allows for bone growth therethrough for the fusion of the adjacent vertebrae. These devices are commonly called “fusion devices”.
U.S. Pat. No. 4,743,256 (“Brantigan”) discloses an improved surgical method for eliminating spinal back pain caused by ruptured or degenerated vertebral discs by spanning the disc space between adjacent vertebrae with rigid fusion devices, or “cages”, having surfaces facilitating bone ingrowth and bottomed on prepared sites of the vertebrae to integrate the implant with the vertebrae and to provide a permanent weight supporting strut maintaining the disc space. Brantigan teaches that these cages are linearly inserted into the disc space from the posterior side of the spine.
Because the Brantigan cage is inserted linearly into the disc space from the posterior side of the spine, its upper and lower surfaces bear against only one of the two sides of the opposing vertebral endplates. For this reason, two Brantigan cages must be used in each surgical procedure.
U.S. Pat. No. 6,143,032 (“Schafer”) discloses an intervertebral fusion device having a banana-shape, including leading and trailing walls connected by a convex wall and a concave wall. This implant may also have a wedge shape wherein the height of the concave wall is smaller than the height of the convex wall. See col. 3, lines 8-9.
U.S. Pat. No. 6,245,108 (“Biscup”) discloses a device comprising a pair of D-shaped cages adapted to fit adjacent one another within the disc space. Each cage has a lordotic anterior-posterior wedge shape, and its curved wall is shorter than its opposite wall so that, in combination, the device provides a dome shape.
U.S. Pat. No. 6,387,130 (“Stone”) discloses providing a plurality of implants which when arranged sequentially produce a banana-shaped device which rests on the anterior half of the disc space. Each implant may have a lordotic shape, as in FIG. 5, and the plurality of implants may be tapered for distraction and lordosis, as in FIG. 6.
PCT patent Publication No. WO 01/28469 A2 (“Frey”) discloses an intervertebral fusion device having a banana-shape, including leading and trailing walls connected by a convex wall and a concave wall. The Frey cage is inserted non-linearly into the disc space from the posterior side of the spine, so that the leading wall thereof comes to rest on one side of the spine, and the trailing wall comes to rest on the other side of the spine. Because the Frey cage bears against each side of each opposing endplate, only one Frey cage need be used in each surgical procedure.
However, Frey discloses positioning the Frey cage in an essentially lateral orientation about midway between the anterior and posterior ends of the endplates. Because the rim of the endplates provides the most stable bearing surface, the Frey implant must have a width that extends across the width of the endplate. Typically, the width of the such cages is about 32 mm.
In addition, the upper bearing surface of the Frey implant has a single inter-end support 1019 connecting the anterior and posterior walls of the implant. Because there is only a single support, the cage is susceptible to rocking about this single support.
In addition, although Frey discloses a pair of insertion holes at either end of the implant, the geometry of the insertion holes appear to be symmetric about the midline of the cage. Accordingly, this symmetric hole placement does not provide the surgeon with any intra-operative flexibility to adjust for differences in patient anatomy or approach.
In addition, although Frey discloses that the anterior wall can have a height greater than the posterior wall, Frey does not disclose the posterior wall can have a height greater than the anterior wall.
Lastly, although Frey discloses that the upper and lower bearing surfaces can have grooves therein, Frey does not disclose that the upper and lower bearing surfaces can have teeth thereon.
PCT Published patent application No. WO 01/70144 (“Scolio”) discloses a banana-shaped implant having three vertically-disposed through holes defining two internal planar walls therebetween. The implant further has a concave wall having a plurality of openings disposed therethrough. Lastly, the implant has a lordotic anterior-posterior wedge, as well as front part 3 to rear part 4 angle. FIG. 7 of Scolio discloses a similar implant having two vertically disposed holes. It appears that the geometry of this cage (lordosis and a medial-lateral slope) requires that it be used to support only one half of the disc space, as with the Brantigan cage.
PCT Published patent application No. WO 02/17823 (“Kim”) discloses a banana-shaped implant having two vertically-disposed through-holes defining a single internal planar walls therebetween. The implant further has a concave wall and a convex wall, each having a plurality of openings disposed therethrough. The upper and lower bearing surfaces of the implant have pyramidal teeth disposed thereon. Lastly, the implant has a lordotic anterior-posterior wedge, as well as front part 3 to rear part 4 angle. FIGS. 10A-E disclose placing the implant on the anterior half of the disc space with its convex wall facing anteriorly.
The Kim cage has a single insertion hole, and so does not provide for surgeon flexibility as discussed above. In addition, it has only a single middle strut, thereby raising the possibility of tilting in the medial-lateral direction. Lastly, the requirement that it have a constant polygonal cross-section precludes the possibility of doming.
U.S. Published patent application 2002/0055781(“Sazy”) discloses a banana-shaped implant having a mesh structure. FIG. 7 of Sazy discloses the implant as positioned essentially in the middle of the disc space.
U.S. Published patent application 2002/0077700 (“Vargas”) discloses a banana-shaped non-porous implant. Paragraph 0055 of Vargas teaches to set the implant as far anteriorly in the disc space as possible.